In many industrial applications, an apparatus that is designed to operate at cryogenic temperatures is located within an insulated container to prevent heat leakage from the ambient to the apparatus.
An example of apparatus that has operational temperature requirement is a cryogenic distillation apparatus in which air is compressed, purified and then cooled to a temperature at or near its dew point for distillation in one or more distillation columns to separate lighter components such as nitrogen and argon from heavier components such as oxygen. The incoming air is cooled against product streams such as nitrogen and oxygen within a main heat exchanger.
Another example is a device for liquefying natural gas whereby gas from a high pressure pipeline is expanded, cooled and condensed to produce a liquefied natural gas (LNG) product.
In order to maintain the low temperatures required for a cryogenic distillation apparatus, a distillation column or columns and the heat exchanger can be placed within a container known as a cold box. Such a container operates at a positive pressure, that is, the container is not sealed to the ambient. Bulk fill insulation, ordinarily in particulate form is introduced into the container to provide insulation. Such bulk fill insulation, for example PERLITE, inhibits convective heat transfer and constrains the heat transfer to take place through conduction. Since conductive heat transfer predominates, radiant heat transfer effects are minimal.
Another type of insulation that has been proposed for use in connection with cryogenic equipment is aerogel insulation. Aerogels have the advantage of having a lower thermal conductivity than traditional insulation materials such as Perlite. Aerogels are water-free gels that are dried such that the solid matter of the gel remains intact to produce an open cell structure which can include inorganic aerogels that are formed of silica, alumina, zirconia, tungsten and titanium. Additionally, organic aerogels such as resorcinol-formaldehyde aerogels have also been formed. Aerogels can be formed as a solid block of material, as a fine powder, or as pellets. Aerogel materials can also be used as fill for a blanket or mixed and strengthened with fibers to form a blanket-like or mat-like structure.
A minimum insulation thickness is required to prevent excessive heat leakage, which will result in local ice spots from forming on the surface of the container. The container is not suitable for exposure to cryogenic temperatures. A minimum thickness is also required to prevent embrittlement of the container walls and structural supports. As can be appreciated, the lower the thermal conductivity of the insulation, the smaller the minimum thickness of insulation, and the smaller the container because less insulation would be required. The problem with replacing a bulk fill insulation material, such as PERLITE, with an aerogel is that although an aerogel has a thermal conductivity that is approximately 4 times less than PERLITE, it is also much more expensive. Therefore, although the containment, for example a cold box, can be made smaller than by the use of PERLITE alone, it has been found by the inventors herein that the added expense of the aerogel insulation does not justify its use in such applications.
As will be discussed, the present invention provides an arrangement of insulation in which the use of low thermal conductivity insulation, such as aerogel is minimized to allow the container to be made smaller than would otherwise be possible with the use of higher thermal conductivity insulation such as PERLITE.